The market for digital projectors for data or video projection has been growing strongly worldwide since the invention of DLP technology. This increasing expansion is also based, inter alia, on the fact that the projectors can be produced at ever more favorable prices because of the technologies used, and can be designed with such compact dimensions that they are capable of mobile use in normal office activity.
The optical system (light engine) of digital projectors usually has a microdisplay panel that in principle constitutes a chip on the surface of which controllable pixels are arranged. The panel is illuminated by the optical system of the projector, and the resulting image is projected onto the wall by means of the projection objective. Three main types of microdisplays are distinguished in principle: DMD, LCD, LCoS. The miniaturization of the projectors depends, inter alia, on the size of these microdisplay panels, and also of the light sources used. The basic design of a projector with an LCD panel is described, for example, in U.S. Pat. No. 5,902,033, while reference may be made with regard to the function of the DMD panels to the relevant patent applications of Texas Instruments, which are directed to the DLP method (digital light processing®).
The abovedescribed digital projection systems predominantly make use as light source of high-pressure discharge lamps such as are described, for example, at www.osram.com under video and projection lamps (VIP lamps). These high pressure discharge lamps have a burner that is inserted into a glass reflector. When designing such high pressure discharge lamps, it must be borne in mind that neither the reflector nor the burner are thermally damaged at the very high temperatures that occur. These burners generally have a discharge chamber that is arranged approximately in the middle and merges into two burner shafts arranged diametrically relative to one another. Because of the comparatively large axial length of these burners, a portion of the output radiation is reflected onto the end section of the shaft such that the latter is exposed to a strong thermal load. It is proposed in U.S. Pat. No. 6,727,650 to provide the end section of the shaft with a reflection layer that acts as a heat insulation layer and reflects the incident radiation such that it is possible to avoid additional heating of the burner in this region. Alternatively or additionally, it is possible to feed cooling air, but the fans required for this purpose increase both the overall space of the projector and its noise emissions.